This invention relates generally to the production of sodium hydrosulfite and, more particularly, to an improved process and system for sodium hydrosulfite generation.
Sodium hydrosulfite, Na.sub.2 S.sub.2 O.sub.4, also known as sodium dithionite, is extensively used as a bleaching agent in the paper and textile industries, and has a wide range of other uses. Because it is relatively unstable, it is generally produced in situ at the point of use, for example in a pulp mill.
Past methods used for producing sodium hydrosulfite have included dissolving zinc in a solution of sodium bisulfite and precipitating zinc-sodium sulfite with milk of lime to leave the hydrosulfite in solution, and reacting sodium formate with sodium hydroxide and sulfur dioxide.
More recent processes include mixing caustic soda and sulfur dioxide with sodium borohydride in an aqueous medium to produce an aqueous solution of sodium hydrosulfite. The sodium borohydride generally enters the process in a mixture with aqueous sodium hydroxide. This mixture, obtainable from Morton International, Inc under the registered trademark "BOROL" has excellent stability because acid hydrolysis of the sodium borohydride is greatly minimized. The sodium borohydride-containing mixture typically comprises 10-15 wt. % sodium borohydride, 35-45 wt. % sodium hydroxide, and 40-55 wt. % water. A typical mixture comprises 12 wt. % sodium borohydride, 40 wt. % sodium hydroxide, and 48 wt. % water. For convenience, this type of process will be referred to hereinafter as the BOROL process.
The theoretical reaction of the BOROL process, assuming ideal conditions and 100% yield, would be as follows: EQU NaBH.sub.4 +8NaOH+8SO.sub.2 .fwdarw.4Na.sub.2 S.sub.2 O.sub.4 +NaBO.sub.2 +6H.sub.2 O
There is, however, a side reaction in which sodium borohydride is hydrolyzed: ##STR1##
This side reaction is a function of pH, with the rate of the side reaction increasing with reduced pH, and acts to reduce the overall efficiency of the process. In practice, however, this competing side reaction cannot be overcome simply by raising the pH as higher pH would adversely affect the main reaction.
The desired reaction, to produce sodium hydrosulfite, can be viewed as effectively taking place in two stages, as follows:
(a) a reaction between sulfur dioxide and caustic soda to give sodium bisulfite (1); and
(b) a reaction between the bisulfite and sodium borohydride to give sodium hydrosulfite (2). EQU 8NaOH+8SO.sub.2 .fwdarw.8NaHSO.sub.3 ( 1) EQU 8NaHSO.sub.3 +NaBH.sub.4 .fwdarw.4Na.sub.2 S.sub.2 O.sub.4 +NaBO.sub.2 +6H.sub.2 O (2)
There is also an equilibrium (3) between the bisulfite and sodium sulfite, which is a function of the pH: ##STR2##
The pH flow profile becomes significant when related to the hydrolysis rate of the NaBH.sub.4. As can be seen by referring to Table 1 set forth below, the rate at which NaBH.sub.4 undergoes hydrolysis increases dramatically as the pH of the NaBH.sub.4 -containing solution decreases. For example, the half-life of NaBH.sub.4 is about 3000 times longer at a pH of 6.0 compared to that at a pH of 2.5. Therefore, if the BOROL solution is added to a process stream having a lower pH than that of the BOROL solution, the extent to which the NaBH.sub.4 in BOROL solution undergoes undesired side reaction, e.g., hydrolysis to form NaBO.sub.2, typically increases, with the rate of hydrolysis typically increasing dramatically as the pH of process stream is decreased. Consequently, when the BOROL solution is added to a process stream of lower pH e g , having a reaction pH of about 5.5-6.5 (such as in U.S. Pat. Nos. 4,788,041 and 5,094,833, for example) or even more so a pH in the range of about 2 to 3, the extent of the hydrolysis of the NaBH.sub.4 is increased.
TABLE 1 ______________________________________ NaBH.sub.4 HYDROLYSIS NaBH.sub.4 pH HALF-LIFE ______________________________________ 2.5 0.00012 SEC 4.0 0.0037 SEC 5.0 0.037 SEC 5.5 0.12 SEC 6.0 0.37 SEC 6.5 1.2 SEC 7.0 3.7 SEC 8.0 36.8 SEC 9.0 6.1 MIN 10.0 61 MIN 12.0 4.3 DAYS 13.0 42.6 DAYS 14.0 430 DAYS BOROL of: 12 wt % NaBH.sub.4 14,000 YEARS 40 wt % NaOH 48 wt % H.sub.2 O ______________________________________ TEMPERATURE = 25.degree. C. (77.degree. F.) LOG (t .sub.1/2) = pH (0.034 T 1.92) t = MINUTES T = .degree.K.
All such sodium hydrosulfite generating processes generally operate within a pH range of about 5 to about 7 and within which pH range the lowering of the pH will generally favor the formation of bisulfite.
Consideration of this equilibrium, therefore, has to be weighed against that of acid hydrolysis discussed above to determine the optimum pH for the process. In such prior processing, a pH of 6.5 has been found to give the best yield. Nevertheless, it has proved difficult to achieve yields greater than about 85%.
In one previous sodium hydrosulfite generation process, SO.sub.2, water, sodium hydroxide (NaOH), and a sodium borohydride/sodium hydroxide/water mixture (BOROL) are fed in that order into a flow line which leads to a static mixer and then to a degassing tank where entrained gases are vented to the atmosphere. An aqueous solution of sodium hydrosulfite is pumped from the degassing tank, a portion of which is delivered to a storage tank for use as required and the balance of which is recirculated to the flow line at a position downstream of the SO.sub.2, water and NaOH inlets but upstream of the BOROL mixture inlet. The input of each reactant can be controlled automatically in response to rising or falling levels in the degassing tank or the storage tank or changes in pressure, flow rates, and/or pH.
Commonly assigned U.S. Pat. No. 4,788,041 depicts an improvement to the above-discussed process. This improvement obtains higher sodium hydrosulfite yields through variations in the proportions of chemicals, pH and temperature measurement and control, and specific changes in the recirculation system.
Commonly assigned U.S. Pat. No. 5,094,833 also relates to an improved process and apparatus for producing sodium hydrosulfite in higher yields. In one embodiment disclosed therein, an inverse order of addition of raw materials, i.e., a first mixture comprising sodium borohydride, sodium hydroxide, and water is introduced prior to a second mixture comprising water and sulfur dioxide, is used to achieve such desirable improvement.
FIG. 1 depicts a typical pH flow profile through the mixing and reaction zones of such sodium hydrosulfite generation process streams. In the prior art as illustrated in FIGS. 2 and 3, mixture of water and SO.sub.2 and solution of sodium hydroxide are added to the process stream upstream of the addition of the BOROL solution (NaBH.sub.4 /NaOH/H.sub.2 O) thereby creating an acidic medium having a pH of about 2.5, into which the BOROl solution is added. similarly, referring to FIG. 4, the solution of sodium bisulfite and sulfur dioxide when added to the process stream creates an acidic medium (pH 2.5) to which the BOROL solution is added. After the BOROL solution is added to the process stream, the pH of the process stream increases to the desired reaction pH. This pH flow profile is illustrated in FIG. 1 by the line graph labeled "Prior Art #1".
In accordance with the processing disclosed in U.S. Pat. No. 5,094,833, discussed above, the BOROL solution and NaOH solution are added to the process stream upstream of the addition of the mixture of water and sulfur dioxide, thus creating an alkaline medium (pH=13.0) into which the mixture of water and sulfur dioxide is added. After the addition of the water and sulfur dioxide mixture, the pH of the process stream decreases to the desired reaction pH. This pH flow profile is illustrated in FIG. 1 by the line labeled "Prior Art #2".
While the processing disclosed in U.S. Pat. No. 5,094,833 results in the generation of sodium hydrosulfite in high yield as compared to prior art processing, the generation of sodium hydrosulfite in still greater yields is desired.